"if 52% of a country votes to intern Muslim immigrants without trial, that does not make it a morally justifiable action".
Exactly, which is why we have representational democracy not direct democracy, with delegates who are a step more informed than Alf Garnett or Bertie Wooster. 500 out of 650 MPs voted Remain, not because they represented the 52% but because they thought about it rather more deeply. They are what rampant Brexiteers call "the elite" and we need them else we'd nuke the next country who annoyed us. In that sense they are profoundly undemocratic. Good.

" three years on now that everyone knows a lot more about the downsides of brexit; it's surely a big enough decision to at least warrant double-checking."
Definitely. They change, that's why we have elections. But not second referendums it seems. And the difference is?
I think the biggest mistake MPs make, and not just Brexit ones, is to cling to a 3 year old marginal instruction that they know is out of date and rendered invalid by events. "Democracy" is being clung to when logic dictates it shouldn't be.

"That said -- and I guess this isn't a massively popular thing to say -- I'd rather the UK government just unilaterally cancelled the whole thing."
Yep! And yes to so much more you said.